In the United States, at least 22845 extremely low birth weight infants (ELBW, birth weight<1000 g) were born in the most recent year for which data are available. These infants account for a significant percentage of the mortality associated with prematurity, and suffer the majority of short and long-term morbidities, including poor postnatal growth. In fact, most of these infants are below the 10th percentile for weight at discharge when compared to intrauterine reference fetuses of similar gestational ages. Several investigators have demonstrated that postnatal growth is influenced by the severity of coexisting morbidities that affect infant nutritional and metabolic status. At least two previous publications (e.g., Ehrenkranz et al., Wright et al., etc.) showed that infants without major morbidities grew at a significantly faster rate than the less healthy infants. Another publication, (e.g., Radmacher et al.) reported a significantly faster rate of growth in infants without bronchopulmonary dysplasia (BPD) than for those with BPD.
Thus, the measurement of postnatal growth is central in the clinical care and research for ELBW infants because it provides an indirect measure of overall infant health, nutritional adequacy, and care practices. Although various growth measurements are used in clinical practice, growth velocity (GV, g/kg/d) is the most frequently reported measure in growth research in ELBW infants. Average GV is an attractive measure for research purposes because it summarizes the infant's weight gain over a desired time interval, often smoothing the variability seen in daily weight measures. Average GV is used to guide day-to-day decisions in the care of ELBW infants, such as determining the feeding regimen. Postnatal GV is also used frequently in research as a dependent variable to assess the safety and efficacy of interventions, particularly nutritional regimens, as well as an independent variable to predict important outcomes such as neurocognitive development, IQ, and the risk of adult-onset diseases.
However, the calculation of GV on a daily basis from actual weight measurements and averaging those measurements over the desired time interval is extremely labor-intensive. Moreover, the ability of many physicians to calculate GV is limited by their unfamiliarity with scientific calculators. Therefore, various differing methods for estimating average GV have been employed in studies that have reported GV as a dependent variable. Some investigators used absolute weight gain over a specified time interval, without normalizing data for either initial infant weight or time. Others used average daily weight gain calculated from weight measurements over a specified time interval, without normalizing data for initial infant weight. Other researchers estimated average GV through the use of linear regression equations that predicted weight as a function of time. Still others reported GV but did not specify the formula used for estimation. Finally, some investigators used z scores to compare birth weight (BW) and postnatal weight at specified times with in utero weight at similar gestational age. With such widely varying procedures for estimating average GV, the results of therapeutic practices cannot be compared across settings and studies.
These different mathematical models yield varying estimates of average GVs, compared with actual GVs. The magnitude of error in estimating GV with these methods was dependent on the starting point and time intervals of the measurement. There are large percentage differences between estimated GV and the accurate standard GV with the 2-point BW and linear BW models and moderate differences with the 2-point average weight and linear average weight models.
The discrepancy in results obtained through these various methods makes comparisons across studies difficult to perform. Although consistently applying the same starting point and time interval may facilitate comparisons, it may not be an appropriate option because the use of each starting point has its own merits. Evaluation of growth starting from birth includes a gross assessment of disease severity, because sicker infants grow more slowly and regain BW later. In contrast, evaluating growth starting from regaining BW measures the adequacy of the nutritional regimen, with the caveat that chronic and late-onset illnesses (e.g., sepsis or chronic lung disease) may affect this growth rate. Therefore, it may be difficult to standardize the starting point and time interval, because these would vary with the particular research question. Moreover the accuracy of these methods of estimating average GV is affected by factors observed commonly among ELBW infants, such as BW, chronic lung disease and longer hospital stays. Therefore, there is a great need for an accurate and uniform method of estimating average GV.